164 Prof. Magnus's Hydraulic Researches. 



smaller, say 2 or 3 centimetres, swellings are produced by every 

 note sounded in the neighbourhood of the jet. Under such cir- 

 cumstances even the human voice is sufficient to produce them. 

 Very high notes alone seemed incapable of causing swellings ; 

 at least by very high notes I was unable to form them in a jet of 

 5 millimetres diameter. 



12-1'. If the pressure under which the water flowed out was so 

 great that swellings were not produced by any but the original 

 note of the stream, or tones standing in simple relation to it, 

 such swellings were nevertheless caused by bringing the sound- 

 ing body into immediate contact with the vessel from which the 

 water flowed, or with the stand on which the vessel rested. 



125. For producing the notes, I have made much use of a 

 small electro-magnet, the keeper of which was fastened to a spring 

 whose motion alternately broke and completed the circuit. This 

 arrangement, known as " NeePs hammer," or the magnetic 

 hammer, may be so adjusted that the motions of the spring and 

 keeper correspond to certain tones which, at least within certain 

 limits, may be varied at pleasure. 



126. Sometimes, instead of this apparatus, I employed differ- 

 ent tuning-forks, especially one very large one which sounded C 

 below the line. 



127. The magnetic hammer, however, has the advantage of 

 producing one and the same note for a long time with undimi- 

 nished strength, thus allowing precise observation. Moreover, 

 in employing it, no mechanical force is necessary, as is the case 

 with stringed instruments, organ- pipes, &c., which force, when 

 the sounding body is in contact with the vessel from which the 

 water flows, or with its stand, imparts a swinging motion to the 

 latter, and consequently to the entire jet. 



128. When the magnetic hammer produced a difi'erent note 

 from the original one of the jet, or one not simply related to 

 it, no swellings were observed as long as the hammer was held 

 in the hand, however nearly it might be held, during vibration, 

 to the stream. But on laying the hammer upon the stand upon 

 which the vessel stood from which the water flowed, the swellings 

 were immediately produced. They appeared with especial sharp- 

 ness and distinctness when the hammer was firmly screwed down 

 upon this stand* For, when this was the case, the vibrations 

 became moi'C I'Cgular, and the note less grating and harsh, than 

 when the hammer was simply pressed upon the stand with the 

 hand. 



Similar phsenomena were produced by the use of the tuning- 

 fork (§ 126). If the fork were struck and held in the hand, 

 no change was produced in the jet, unless the note of the fork 

 accidentally coincided with the original tone of the stream. No 



